My invention relates to polishing materials such as silicon wafers generally used in producing semiconductors. In particular, my invention involves an improvement to the process of polishing silicon wafers with a polishing agent consisting of colloidal silica and an amine.
Silicon wafers sliced from single crystal ingots require several polishing steps to provide the flat defect-free surface required for further processing. Such polishing is best accomplished using any of several machines designed for the purpose and an aqueous slurry or dispersion of a polishing agent. The industry has preferred silica sol for polishing. Walsh and Herzog (U.S. Pat. No. 3,170,273), among others, teach the use of silica sols for the purpose. Payne in U.S. Pat. No. 4,169,337 discloses an improvement to this process by the inclusion of up to 5% of an amine in the sol. In U.S. Pat. No. 4,462,188 Payne discloses an improvement to his own process, said improvement consisting of the addition of a quaternary ammonium salt or base to the amine silica-sol combination. These teachings can be construed to provide a silica sol that contains up to 5% of an amine and up to 5% of said quaternary, these percentages being based on the SiO.sub.2 in the sol.
The industry tends to practice the polishing process with sols such as those taught by Payne as follows. The sol is received at about 30 to 60% SiO.sub.2 with the required amine added thereto. The sol is diluted to about 3% SiO.sub.2 (10 to 20 times) with water. The pH of the diluted sol is adjusted to 10.5 to 11 with alkali metal or ammonium hydroxide as taught by Payne and others. Potassium hydroxide is preferred, although not ideal. NaOH and NH.sub.4 OH have more distinct disadvantages. The higher pH is required to improve stock removal. See Basi, et al., U.S. Pat. No. 4,549,374. The sol should be used soon after dilution and pH adjustment, because the pH tends to drop on aging. The pH also decreases as polishing proceeds. Both these situations require that additional KOH be added to establish and/or sustain the desired stock removal rate.
Polishers would consider a sol-amine combination that requires only dilution avoiding pH monitoring and adjustment a distinct and valuable improvement to the process. To provide such a combination, additional alkaline materials would have to be added to the concentrated sol. Addition of substantial amounts of alkaline materials to concentrated sols has not been thought a reasonable approach, since the chemistry of silica sols does not encourage such additions. Silica sols that contain more than 20% SiO.sub.2 are carefully balanced colloidal systems stabilized by a combination of structured water layers surrounding the silica particles and a limited number of counter ions. The addition of alkaline bases or salts destabilizes these sols by disrupting the structured water layers and by the alkali attacking the silica polymer. These conditions cause the sol to become progressively more viscous and finally to gel.
It is an object of this invention to improve the polishing of silicon metal wafers by employing a silica sol-amine combination that contains sufficient amine to provide a high pH on dilution and that said silica sol-amine combination is stable. It is a further object of this invention to eliminate addition of alkali metal prior to and during the polishing process. The alkali metal present is thereby limited to any that is required during sol preparation.